This invention relates to an antenna apparatus and, in particular, to an antenna apparatus for use in reception of a radio signal from an artificial satellite, i.e., a satellite signal.
In recent years, a digital radio receiver adapted to receive a satellite signal has been developed and is put into practical use in United States of America. The digital radio receiver is mounted on a mobile object such as an automobile and is adapted to receive a radio wave having a frequency of about 2.3 GHz as a reception radio wave. In other words, the digital radio receiver is a radio receiver capable of listening to mobile broadcasting. Since the reception radio wave has a frequency of about 2.3 GHz, a reception wavelength (resonance wavelength) λ is equal to about 128.3 mm. It is noted here that the satellite signal is not only transmitted as a satellite wave but also as a ground wave derived from the satellite wave. The ground wave is obtained by slightly shifting the frequency of the satellite wave after the satellite wave is received by a ground station and is retransmitted with linear polarization.
Since the satellite signal is transmitted by the use of the radio wave having the frequency of about 2.3 GHz, an antenna for receiving the satellite signal must be located outdoors. In case where the digital radio receiver is mounted to the automobile, the antenna is generally attached to a roof of the automobile. On the other hand, a reception apparatus such as a receiver is disposed inside the automobile and connected to the antenna through a lead wire.
Typically, the automobile is equipped with an FM receiver for receiving a radio signal in an FM band. Therefore, in order to receive the satellite signal, an independent digital radio receiver must be equipped in addition to the FM receiver. Further, in some cases, an additional loudspeaker will be required to be installed. Therefore, reception facilities mounted to the automobile as a whole are complicated, resulting in an increase in cost.